Frequency-modulation echo-sounder



June 2, 1964 D. G. TUCKER ETAL 3,135,942

FREQUENCY-MODULATION ECHO-SOUNDER 3 Sheets-Sheet 1 Filed may 11, 1960June 2, 1964 D. G. TUCKER ETAL FREQUENCY-MODULATION ECHO-SOUNDER 3Sheets-Sheet 2 Filed May 11, 1960 .m .o SQ l .R2 1m l1. S m M M y w GUnited States Patent 3,135,942 FREQUENCY-MDULATIGN ECH-SOUNDER DavidGordon Tucker and Leslie Kay, Birmingham, Engiand, assignors to NationalResearch Development Corporation, London, Engiand, a British corporationFiled May 11, 1969, Ser. No. 28,317 Claims priority, application GreatBritain May 12, 1959 9 Claims. (Cl. 340-3) Fish swimming close to thesea-bed are very difficult to detect by conventional echo-sounding orecho-ranging devices because the echo returns from the fish are almostindistinguishable from the sea-bed returns in the case of echo-ranging,and variations in the depth of water, irregularities of the sea-bed andthe vertical motion of the ship restrict the degree of resolution whichcan be obtained in the region of the sea-bed on an echo-soundingrecorder.

I The present invention is designed to present echoes from fish verynear the sea-bed as clearly discernible from the sea-bed echo. Theequipment may be designed to to detect fish within 2 or 3 fathoms of theseabed, and the presentation may be aural, thus using a sense notnormally exercised while fishing.

Known echo Sounders use the instant of transmission as the datum fromwhich the echo time is measured to give an indication of depth. In deepwater, up to say 300 fathoms, variations in depth are usually only avery small fraction of the total depth, and 2 or 3 fathoms change indepth are hardly noticeable on a record. Fish Within a few feet of thebottom will not be detectable unless the region near the sea-bed isdisplayed on an expanded scale, but to do so is difficult because thedepth may vary by Very much more than the scale covered on the recorder.Devices which lock the datum of the display to the bottom echo are knownand enable the display to be expanded without reference to depthvariations. The present invention is concerned with a new approach tothe problem, enabling fish near the bottom to be detected and located ina relatively simple manner.

According to this invention, an apparatus for detecting the presence offish near the sea-bed comprises means for transmitting afrequency-modulated exploring signal, means for detecting echo signalsreceived from the seabed and objects separate from and within a givendistance from the sea-bed and means for setting up beat signals betweensaid echo signals to provide an indication of the presence of areflecting object, other than the seabed within said given distances.Means are preferably provided for rendering said beat signals audible togive an indication of the distance from the sea-bed of the reflectingobjects by reference to the frequency of said beat signals. Theequipment may also be arranged to provide a beat signal between thebottom echo signal and the transmitter signal to give an indication ofdepth by reference to the frequency of this beat signal, and this signalmay also be made audible.

The invention will be more clearly understood from the followingdescription of an embodiment thereof given with reference to theaccompanying drawings in which:

FIGURE 1 is a block schematic diagram of a complete system according tothe invention,

FIGURE 2 is a graph used in explaining the operation of the system andFIGURE 3 is a detailed circuit diagram of part of thesystem of FIG. 1.

Referring first to FIGURE 1, it will be seen that the transmitterconsists of a frequency-swept oscillator 1 controlled by a saw-toothgenerator 2, and a power amplifier 3 which drives the electro-acoustictransducer 4 to produce ultrasonic waves in the water. The frequency oftransmission is shown (as a typical example) to vary from 340 kc./s. to460 kc./s. in periods of l second, 0.5 second or 0.25 second intervalsfor depths of fathoms, 30 fathoms and 15 fathomsv respectively. (It willbe seen later that the wide band and long duration of sweep is used togive an almost continuous audible note when an echo is present.) Fordeeper water, a lower frequency of. transmission would be used -with acorrespondingly slower frequency sweep rate. The transmitting transducerwill be mounted in a typical system on the keel of the ship lookingdownwards. The transducer may, for example produce as acoustic beam ofof the order of 3 width. Since the transmission is continuous, only alow power is required as compared with a pulse system.

A receiving transducer 5 will be mounted near the transmittingtransducer and receive more-or-less con,` tinuous signals fromreflecting objects at any depth. Modulating these input signals to thereceiver with the direct signal from the EM. oscillator, a sum and adierence frequency corresponding to each input signal will be obtainedat the output of the frequency changer 6.

FIGURE 2 is a diagram of frequency versus time, curve (a) being thetransmitted signal, curve (b) the bottom echo signal and curve (c) thesignal due to, say, a fish shoal near the bottom. From FIGURE 2 it willbe clear that each difference frequency output from 6 is proportional tothe depth from which'that signal Was reflected back to the transducer.Only these difference signals are used in the system; the sum signalsare rejected by the band-pass lter 7.

Since the intensity of the received sound waves decreases with depth, anequalizer 8 and amplifier 9 are used following filters 7 to correct forthe fall in amplitude with increasing frequency at the output of thefrequency changer.

Because the system is` being used as an echo-sounder the dominant echowill be that from the sea-bed, and this will be very much greater inamplitude than any other echo. Short range echoes may be stopped ifnecessary by the filter 7. The difference signals which may be termedprimary beat signals are applied to a detector (or rectifier) 10, theoutput from which will be a pulsating D.C. signal that may be regardedas being composed of not only a D.C. due to the rectified bottomechosignal (and the latter signal itself, and its harmonics, if only asimple rectifier circuit is used), but also low frequency secondary beatsignals produced by the beating together of the bottom-echo signal andany other echo signal arising, e.g., from a fish or fish shoal near thesea-bottom. Filter 11 is designed to reject the D C. and

- the higher frequencies referred to above, and has its passband chosenso that only the low-frequency secondary beat signals derived fromecho-signals arising originally in the sea immediately above the bottom(say within a range of 2 or 3 fathoms above the bottom) are passed tothe next stage. The bottom echo is thus used as a carrier in operatingthe detector.

The important effect is that now the bottom echo, no matter what thedepth, is a D.C. signal and becomes the datum to which the low-frequencysecondary beat signals must be referred. Hence variations in the depthand vertical movement of a ship are eliminated because the system outputis locked to the sea-bed. This is due to the fact that vertical movementof the ship causes alteration of the primary beat signals from thesea-bed and from other reflecting sources by equal amounts and hence noalteration in the resulting secondary beat signal.

The filter 11 is used to stop the D.C. and in this example all signalsabove 700 c./s. which corresponds to 2 fathoms. A wider filter bandwould obviously cover a greater range of depths above the sea-bed.

For ease of presentation the output of the filter 11 is frequencychanged in mixer 12, fed'with 1 1ro/s. from the local oscillator 13, to1040-1700 c./s. which is Within the optimum audible range of the ear,and is applied through filter 14 and amplifier 15'to loudspeaker 16; K K

Any echo signal which originates in the two fathoms just above thesea-bed will appear in the'outputV loudspeaker-or headphonesas amore-or-less constant tone.

There will be no background-as in an ordinary echo-ranging device, sinceall signals except `the required secondary Y beat signals have beeneliminated by filters 7, 11 and 14 and thus the tones will be clearlyaudible and immediately indicate the presence of fish, the pitch of theAtone giving an indication of the depth of the fish; the higher thefrequency the further from the bottom are the fish.

The audio amplifier 15 can beV connected to the output from amplier 9,by reversing switch 17, to obtain a tone indicative of the depth to thebottom. At the same time a K change in the band of frequency wouldV haveto beV made in order Vthat the bottom echo wasV audible (say -8`kc./s.). v f Y K .This is effected in mixer to which is applied a signalfrom oscillator 19 which may, if desired, KtakeKthe form of a beatfrequency oscillator, the outputof which may be adjusted to bring themixer output to VVZero frequency if an Vaccurate measure of the bottomsounding is Kre-r quired. A bandv pass filter 24 eliminates unwantedfrequencies which might lead to ambiguity in the depth measurement.Instead of, or in addition to the audible presentation by loudspeaker16, the signal from filter 14'V may be passed througha spectrum analyserKto a visual displayon a cathode ray tube and/ or a recorder indicatedbyl 21. Similarly, the bottom sounding signal from amplifier 9 can beapplied through spectrum analyser 22 to aY visual display or recorder23.

FIGURE 3 shows in detaila circuit suitable for performing the functionsof the depth equalizer, the amplifier 9, the detector 10 and theband-pass filter 11.V The signal fromthe filter 7 is applied to the gridof amplifier Vvalve 25, the output from which is applied to a tunedcircuit, consisting ofcoil L and condenser C1. The voltage across thiscircuit is applied to thecontrol grid of ampli- Vsonicsignal, means forsweeping theffrequency ofsaidK ultrasonic signal through a range Vof.frequencies, means for receiving echo signals, heterodyning meansforKheteroY-K dyning said echo signals and the transmittedsignal toprovide arbeatl signal representing by itsfrequency the de-K lay betweenthe. transmission of a given signal and, recep-` K- tion of thecorresponding echo signal, heterodyning means. y

for heterodyning the beat signals corresponding to echoes' from theVsea-bed` and thev beat signals corresponding-to echoes .from any otherobject within a given range of the sea-,bed to provide a'secondaryKbeatYsignal and indi-"Kv eating means responsive tothe presence ofsaidsecondary beat-'signal for indicating thepresence ofy 'a reflecting ob-KK' ject other than the sea-bed withinV said range. Y.

2,. Apparatus as claimed in claim 1 wherein said indi-v catingmeansincludes means for converting said second-Q v ary beat signal to afrequency within the audible'K-rangeK and means for rendering theconverted signal audible.

Y heterodyning means which provides said secondary beat" signal. t

KBLApparatusk as claimediKnKclaim l including means Y for selecting kthebeatsignal'derived from Vthe yecho from the sea-bed and providinganKindKication of its frequency to f givea measure of theKsea depth.

4. Apparatus as claimed inclaimfl including means for compensating fordecreasing signal amplitude at increas-fV ing range, said meansincludin'g'frequency sensitive means Y providing increasing signal gainwithmcreasing-frequency in the signal pathKbetween said receiving meansandsaid 5. Apparatus as'claimed in claimL 1 ywherein said indi- Y eatingmeans includes a spectrum analyzer and visual dis- K Vplaymeans- Y K 6.Apparatus as claimed in claim 1 wherein said indit eating means includesa spectrum analyzer and visualire-K cording means. K Y K ,K K 7.'Apparatus forA detecting the presence lof objects near the sea-bedvcomprising meansV for transmitting an ultrasonic signal, means forfreqt'iency-modulatingsaidsignal fier valve 26., The output fromamplifier 26 is fed to a rectifierv 27 which works into a load resistor28, the ksignal developed across which is fed into the band-pass filterconsistingof series-capacitor C2, and series resistors 3i) and 31,parallel capacitors C3 and vC4 and parallel resistors 28 and 29 toprovide a signal output at B. VThe output from B is frequency changed inthe Vmanner describedV Thus, as the range increases they through a rangeof frequencies, means for receiving echoV signals, meansl forheterodyning said echo signals withthe K transmitted signal to providebeat signals the `frequencies of which represent the delay'between thetransmission of' K a given signal Vand reception Yof thecorresponding'echo signal, detector means to which said beat Ysignalsare applied to derive secondary beat signalsthe `frequencies of f KWhichare determined by the frequency difference between the beat signalscorresponding to echoesffrom the'Ksea-bedV and beat signalscorresponding to echoes from any other objectwithin a given rang'eflomthe sea-bed, andmeans for indicatingthe presence ofssaid secondary beatsignals;

' 8.,Apparatus as claimed in claim 7 inwhich said fre-Kquency-modulating means provides a linear Ksawtooth freamplifier 26with increasingY frequency, secured by the im- Y pedance characteristicof its input circuit LCI. The output from tube 26 besides being fed intothedetector 27 is fed. via the tapping on potentiometer VP over line 33and resistor 34 to mixer 18 as above described.

The switch S1A`selects the appropriate oneofJthIee,

resistors 35, 36 and 37 to be connected as damping for the tuned circuitLCI, so as to `match the characteristic of the circuit to the range/frequency characteristic'of the echo sounder.

It can therefore beganged to a switch f controlling the setting of thesweep generator 2, whichy controls the frequency sweep of thetransmittter, so as to K adapt the system for operation over the threedifferent depth ranges referred 'to above.

We claim:

l. Apparatus for detecting the presence of objects near the sea-bedcomprising means for transmitting an ultraquency sweep .y

,9. Apparatus for detecting-the Vpresence offurther objects separatefrom and within ya given distance Vfrom a. firstlobject comprising meansfor transmitting a frequencymodulated exploring signal, means fordetecting echosi'gnals received from said first object and said furtherVobjects, means for heterodyning detected echo signals from said first,Kobject with'detected echo signals'frorn said fur-'K` 4 ther objects toproduce abeat signal, and means for in- Y dicating the presence of asaid beat signal, whereby inforjects within said given distance. s Y

ReferencesCited in the` file 'ofy this patent UNITED; STATES PATENTSYHiggins etfal. l July 9,1940

massek K 2,413,620 Guanella f K. Dec. 31, 2,638,586 Guanella K vMay 12,y1953 2,724,817 I-IisserichV a Nov. 22, 1955 nqs'hon .et ai. Mar. 2s,1961 ,Y

kmation is provided as to the presence of anyKfurtherfob-

9. APPARATUS FOR DETECTING THE PRESENCE OF FURTHER OBJECTS SEPARATE FROMAND WITHIN A GIVEN DISTANCE FROM A FIRST OBJECT COMPRISING MEANS FORTRANSMITTING A FREQUENCYMODULATED EXPLORING SIGNAL, MEANS FOR DETECTINGECHO SIGNALS RECEIVED FROM SAID FIRST OBJECT AND SAID FURTHER OBJECTS,MEANS FOR HETERODYNING DETECTED ECHO SIGNALS FROM SAID FIRST OBJECT WITHDETECTED ECHO SIGNALS FROM SAID FURTHER OBJECTS TO PRODUCE A BEATSIGNAL, AND MEANS FOR INDICATING THE PRESENCE OF A SAID BEAT SIGNAL,WHEREBY INFORMATION IS PROVIDED AS TO THE PRESENCE OF ANY FURTHEROBJECTS WITHIN SAID GIVEN DISTANCE.